The present invention relates to a coordinate measuring machine for determining spatial coordinates on a measurement object, and also to a probe head configured to be used on such a coordinate measuring machine. Moreover, the invention relates to a method of determining spatial coordinates using such a coordinate measuring machine and probe head.
U.S. Pat. No. 6,430,828 discloses a coordinate measuring machine comprising a probe head on which a probe tool having a stylus is arranged. The stylus is attached to the lower free end of a vertically arranged quill. The quill can be moved in a vertical direction such that the probe head can be moved orthogonally to a measurement table used for holding a measurement object. The quill itself is arranged on a cross beam of a gantry, and it can be moved in a first horizontal direction along the cross beam. The gantry can be moved together with the quill in a second horizontal direction such that the probe head overall can be moved in three mutually perpendicular spatial directions. The maximum movement distances of the probe head along the three movement axes define a measurement volume within which spatial coordinates on a measurement object can be determined.
In order to carry out a measurement, the measurement object is arranged on the measurement table. Selected measurement points on the measurement object are then touched with the free tip of the stylus. Spatial coordinates for the touched measurement point can then be determined from the position of the probe head within the measurement volume and from deflections of the stylus relative to the probe head. Geometric dimensions and even the object contour of the measurement object can be determined by determining a plurality of spatial coordinates at different measurement points. One typical field of application for such coordinate measuring machines is the measurement of workpieces for quality control.
The measurement points on a measurement object are often located at a position where access is difficult for the stylus, for example when the depth of a hole arranged at the side of the measurement object is to be determined. In order to access such “concealed” measurement points, it is known to use different probe tools having different styli and/or stylus combinations. For example, there are probe tools in which a stylus is arranged transversely with respect to the spatial z-axis of the coordinate measuring machine. In order to carry out a large number of complex measurement tasks, frequent changes of the styli and/or stylus combinations are often required. This is disadvantageous because a stylus change costs time, and therefore prolongs the measurement time for carrying out the measurement. This is a result in particular of the long movement distance of the probe head, since the probe head is typically replaced at a magazine which is arranged outside the measurement volume. In order to carry out a stylus change, the probe head must be moved out of the measurement volume and must then be returned to the original position. The time required for the change is dependent on the size of the coordinate measuring machine. In addition, flexibility of the available probe tools is restricted, and this leads to a large number of probe tools being required for complex measurements. For example, if the depth of a bore inclined at 45° to the surface of the measurement object is to be determined, a suitable stylus or a suitable stylus combination is required. Furthermore, use of such styli and/or stylus combinations restricts the measurement volume, since the probe head can be moved only until it comes in contact with one of the lateral supports. In the case of long styli, this leads to a major restriction.
U.S. Pat. No. 6,430,828 proposes a coordinate measuring machine having a magazine, which has an apparatus for rotating the probe tool. The rotation is carried out about the spatial z-axis by placing the probe tool together with the stylus and/or the stylus combination in this apparatus, which means it is separated from the probe head, and by rotating it using the apparatus, and then fixing it again to the probe head. This apparatus enables each probe tool to be used in one of several rotated positions. This reduces the number of required probe tools with corresponding styli and/or stylus combinations, since each probe tool can be used in several orientations about the z-axis. This approach also makes it possible to reduce the restriction to the measurement volume caused by the styli. One disadvantage in this case is that the rotation of the probe tool requires that the probe tool is placed into the magazine, as a result of which a long time is still required to move the probe head to the magazine, to place it there and to move it back after picking up again.
DE 101 14 126 discloses a probe head which makes it possible to rotate a probe tool directly on the probe head. For this purpose, a lowering apparatus is provided on the probe head, which lowering apparatus first lowers the probe tool in order to enable rotation. After the lowering process, rotation is carried out by an electric motor, which is provided for this purpose in conjunction with a gearwheel drive. Following the rotation, the probe tool is raised again by the lowering apparatus. One advantage here is the gain in time, since the probe tool need not be moved to a magazine for rotation. This also allows that a counterbalanced state of the probe head can be maintained. One disadvantage, however, is that the probe head must be equipped with a complex lowering apparatus and a motor and gearbox for this purpose. This increases the weight of the probe head, leading to undesirable oscillations on the probe head during movement. Furthermore, this results in the probe head being more complex, resulting in additional complexity in design, production and maintenance.
U.S. Pat. No. 5,185,936 discloses another probe head with a probe tool that can be rotated. The probe tool is magnetically held on the probe head. A holding magnet is arranged at a distance from the probe tool, while the latter is being used. Furthermore, it is connected to a shaft which can be rotated by means of a motor that is provided for this purpose. In order to rotate the probe head, the holding magnet is moved together with the shaft in the direction of the probe tool, as a result of which the probe tool is moved from its position. The probe tool is then rotated by means of the motor. The shaft is then drawn back to its original position, as a result of which the holding magnet is released from the probe tool. The drive motor in the probe head again increases weight and complexity.